Optical disks have been used not only in the video and audio fields but also in the field of computer as document files, external memories and the like. Since semiconductor laser is used for recording and reproducing information on the optical disks, substrates for the optical disks are required to have high heat resistance, high transparency, low water absorption and low birefringence. The optical disk substrates are also required to be produced at high productivity. In order to meet these requirements, various substrate materials are used depending on use of the optical disks: polycarbonate resins (hereinafter abbreviated as PC) are used in compact disks (CD); polymethyl methacrylate resins (PMMA) are used in video disks (LVD); and glass, PC or PMMA is used in optical disks serving as document files.
Erasable-DRAW (direct read after write) optical disks such as external memories for computers which are capable of information recording, erasure and reproduction require substrates exhibiting more heat-resistant, less moisture absorption and less birefringence than the conventional substrates for CD, LVD and document files. In an attempt to satisfy this need, various PCs and low-moisture-absorption PMMAs have been proposed, as described, for example, in JP-A-58-179224, JP-A-60-166322, JP-A-60-130611, JP-A-60-147416 and JP-A-61-36307. (The term "JP-A" used herein means an unexamined published Japanese patent application.) However, PCs are low in molding efficiency and disk substrates produced therefrom experience an increased degree of birefringence. Substrates made of PMMAs are satisfactory with respect to birefringence but they do not exhibit good moisture absorption and good heat resistance at the same time. The use of reinforced glass has also been proposed, but in order to form pregrooves, the so-called "2P" process must be employed and this results in a very low productivity. In addition, the pre-grooved glass substrate is liable to crack. Thus, the substrate materials previously proposed are not completely satisfactory in terms of various physical properties and productivity.